1. Field of the Invention
The present invention relates to a liquid ejection device and a dummy jet method, and more particularly, to a dummy jet for an ink jet head.
2. Description of the Related Art
In some types of ink jet head which use ink (for example, an aqueous pigment ink) using an aqueous solvent, a nozzle portion dries in the following situation and an ejection performance deteriorates.
When the nozzle portion is left unused in a standby cap portion, it is difficult to completely prevent the drying of the nozzle portion even though the standby cap portion has a moisture retention function of suppressing the drying of the nozzle portion.
When the nozzle portion is left unused above transportation means for transporting a recording medium, the nozzle portion dries while the recording medium is being moved from the transportation means to the standby cap portion even though the period for which the nozzle portion is left unused and the period for which the recording medium is moved are relatively short.
During image formation, a nozzle portion which does not eject ink or a nozzle portion which ejects a relatively small amount of ink dries.
As measures for the drying of ink in the nozzle portion, a dummy jet (preliminary ejection, idle ejection, or spitting) is performed as means for, before an ink ejection surface in which nozzle openings are formed is wiped (swept), removing ink whose viscosity has increased due to the drying of the nozzle opening and the vicinity of the nozzle opening and ink which has been attached to the edge of the nozzle opening and then semi-hardened.
For example, the dummy jet is performed for each nozzle about 20000 times to suppress a reduction in the ejection performance. However, mist is attached to the ink ejection surface by the dummy jet.
When a large amount of mist is attached to the ink ejection surface, ink which is ejected from the nozzle openings is combined with mist, which may cause a change in the ejection direction of the ejected ink, or the mist attached in the vicinity of the nozzle opening becomes hardened or semi-hardened, which may cause a change in the ejection direction of the ink ejected from the nozzle opening.
That is, the ejection of a large amount of ink in the dummy jet makes it possible to obtain the effect of suppressing the drying of ink in the nozzle portion, but causes another problem that mist is attached to the ink ejection surface.
When the number of ink ejections in the dummy jet increases, the amount of mist attached to the ink ejection surface increases. In order to remove mist attached to the ink ejection surface, it is necessary to perform a separate process, such as a wiping process, which results in an increase in a maintenance period and maintenance costs. That is, a trade-off relationship is established between the suppression of the drying of ink in the nozzle portion by the dummy jet and the attachment of mist to the ink ejection surface by the dummy jet.
In an ink jet head which ejects black ink, a water repellent film on the ink ejection surface is worn by a carbon black pigment included in the black ink due to the attachment of mist to the ink ejection surface.
When the number of wiping operations which remove the mist attached to the ink ejection surface increases, the abrasion of the ink ejection surface is accelerated by the carbon black pigment, which makes it difficult to increase the durability of a liquid repellent film on the ink ejection surface. Then, the ink jet head (a head module forming the ink jet head) is frequently replaced, which results in a reduction in the operation efficiency of the device due to the replacement of the ink jet head and an increase in costs.
JP2009-45803A discloses a technique which relates to a time-division driving method for an ink jet head (recording head) and reduces the amount of mist attached to an ink ejection surface when a dummy jet (preliminary ejection) is performed.
JP2012-245758A discloses a technique which causes a phase difference between driving pulse signals applied to a plurality of actuators corresponding to a plurality of nozzles and changes the phase difference, depending on the length of a flow path, to reduce the amount of ink discharged in a dummy jet (purging).
JP3155762B discloses a technique which makes a driving frequency during a dummy jet (idle ejection) equal to a maximum driving frequency during ink ejection.